In seismic exploration, an acoustic wave is generated at or near the surface of the earth. The acoustic wave travels downwardly and is reflected from subsurface earth layers, whence the wave returns to the earth's surface. The reflected seismic waves are detected by sensitive seismic sensors distributed in a substantially linear array at or near the surface of the earth.
In accordance with the usual practice up to the present time, 25 to 30 seismic sensors are equally spaced along a single cable section, 100-300 feet long. The sensors are all electrically connected together to form a single elongated data channel. Fifty or more such cable sections are coupled together to form a cable assembly 10,000 feet or more long. The output of each of the 50 or more channels is connected to a central signal processing device located at one end of the cable assembly. A separate pair of wires is connected to each channel. There may be therefore, 50 or more such pairs. Because of the expense and physical weight of such a large number of wires, schemes have been proposed wherein the output signals from each channel are time-delay multiplexed through a single data transmission channel. See for example copending U.S. Pat. applications Ser. Nos. 576,943 and 446,862, now U.S. Pat. Nos. 3,996,553 and 3,990,036.
Although this invention will be described with particular reference to geophysical surveying, it may be used to detect objects submerged beneath the water such as large fish, vessels, and the like.
Other prior art systems useful in reviewing the background of this invention are U.S. Pat. Nos. 3,851,302; 3,652,979; 3,748,638; and 3,873,961. All of these patents teach the use of telemetering systems wherein a signal processing module inside of each of a plurality of seismic cable sections digitizes the seismic data from one seismic detector or detector group. By use of various multiplexing schemes, the data are transmitted to a recording device over a telemeter channel.
As discussed earlier, a seismic sensor group is an array 100-300 feet long. An acoustic wave whose wavefront is substantially parallel to the plane along which the seismic sensors are disposed, will arrive at all of the sensors substantially simultaneously thereby enhancing the output signal by summation. Under actual seismic prospecting conditions this ideal situation rarely exsists, except for the special case in which the wavefront is reflected vertically from a reflector lying directly below the array. In the more usual case, the acoustic wavefront sweeps across the array at an angle. Because of the angularity, the wavefront impinges on one end of the array well ahead of the time that it reaches the other end of the array. The travel time of the wavefront across the array depends upon the seismic wave phase velocity, the wavelength of the acoustic wave, the array length and the depth of the reflector, among other factors. If the array length is large compared to the wavelength of the acoustic wave the array will attenuate the acoustic wave. The seismic sensor group or array must be shorter than the wavelength of the incident seismic waves to avoid attenuation and preferably, the group should be less than one-quarter of this wavelength for good response.
Traditionally, energy in the lower end of the seismic spectrum, involving wavelengths of many hundreds of feet, was of interest. Use of relatively long arrays was satisfactory. More recently, however, greater resolution of geologic layering is being demanded. Greater resolution requires use of seismic waves having higher frequencies. But the wavelengths of the energy in the upper end of the seismic spectrum are comparable to or much shorter than the length of conventional seismic sensor arrays in present use. It would be desirable, therefore, to substitute many shorter seismic sensor arrays instead of one long seismic sensor array in each cable section. Such a possibility has been considered to be impossible in view of the data processing complexities and the large number of conductors that would have been required, resulting in seismic cable assemblies of impractical size and bulk.